1. Field of the Invention
The present invention relates to a liquid droplet ejection head and an image forming apparatus, and more particularly to a technique for arranging wiring for driving a piezoelectric element provided in the liquid droplet ejection head.
2. Description of the Related Art
An inkjet-type image forming apparatus comprises a print head having a large number of nozzles arranged in a matrix form. An image is formed on a recording medium by depositing ink droplets onto the recording medium from the nozzles.
In a print head according to a related art shown in FIG. 18, ink is supplied to a pressure chamber 52 from a common liquid chamber 55 disposed on the same side as the pressure chamber 52, and a diaphragm 56 which forms a ceiling surface of the pressure chamber 52 is the boundaries of the pressure chamber. When an electric signal corresponding to image data is transmitted to a piezoelectric element 58 disposed above the diaphragm 56, the piezoelectric element 58 is driven to deform the diaphragm 56. As a result, the volume of the pressure chamber 52 decreases, causing an ink droplet to be ejected from a nozzle 51. The ink droplet lands on the recording medium, and thus forms a dot on the recording medium. By combining such dots, a single image is formed on the recording medium.
In recent years, demands have been made for improvements in the image quality by image forming apparatuses. To achieve high image quality, the nozzles must be arranged in the print head at a high density to increase the number of pixels per image. Various techniques for increasing the nozzle density have been proposed in the related art (see, for example, Japanese Patent Application Publication Nos. 9-226114, 2001-179973, 2000-127379, 2000-289201, 2003-512211, and so on).
Japanese Patent Application Publication No. 9-226114 discloses a print head in which a piezoelectric element is disposed on a diaphragm constituting the ceiling surface of a pressure chamber, a reservoir (common liquid chamber) is provided on the piezoelectric element side of the diaphragm, and an ink supply hole is provided in the diaphragm.
Japanese Patent Application Publication No. 2001-179973 discloses a print head in which a piezoelectric body (piezoelectric element) is disposed on a diaphragm constituting the ceiling surface of a pressure chamber, and an ink supply tank (common liquid chamber) is provided above the piezoelectric body across a partition wall.
Japanese Patent Application Publication No. 2000-127379 discloses a print head in which a reservoir (common liquid chamber) is formed on the same side as a piezoelectric element that is disposed on an opposite surface side to the nozzle side of a pressure generating chamber (pressure chamber).
Japanese Patent Application Publication No. 2000-289201 discloses a print head in which a piezoelectric actuator (piezoelectric element) and a common ink chamber (common liquid chamber) are disposed on the same surface side as a nozzle side of a pressure chamber, and a substrate (wiring layer) is disposed on the opposite surface side to the nozzle side of the pressure chamber.
Japanese Patent Application Publication No. 2003-512211 discloses a print head in which an ink supply layer made from porous member for supplying ink to a pressure chamber is disposed between a nozzle layer in which nozzles are formed and a cavity layer constituting an ink cavity (pressure chamber). A piezoelectric element is disposed on a displacement plate (diaphragm) which forms the ceiling plate of the ink cavity, and a conductive connecting element (wiring member) is provided from the piezoelectric element in a direction substantially perpendicular to the diaphragm. A substrate (wiring layer) is beyond the conductive connecting member.
In the print head according to the related art shown in FIG. 18, the flow passage connecting the common liquid chamber and pressure chamber has a complicated constitution. Therefore, when highly viscous ink is used, a problem arises in that the refilling performance to supply ink to the pressure chamber following ink ejection is not good.
When the wiring for driving the piezoelectric element is arranged on the diaphragm, as in the case of the print heads disclosed in Japanese Patent Application Publication Nos. 9-226114 and 2001-179973, it is difficult to secure sufficient space for the drive wiring and to dispose the nozzles at a high density.
In the print head disclosed in Japanese Patent Application Publication No. 2000-127379, the drive wiring for the piezoelectric element is formed by wire bonding or film deposition, and connected to external wiring mounted above the common liquid chamber. However, since the drive wiring is provided on the exterior of the common liquid chamber, it is difficult to secure sufficient space for the piezoelectric element drive wiring, and restrictions are also placed on the size of the common liquid chamber. When the size of the common liquid chamber is reduced, the ink supply to each pressure chamber tends to be insufficient, and hence it becomes difficult to drive each nozzle at high frequency. Moreover, Japanese Patent Application Publication No. 2000-127379 only deals with the constitution of a print head having a single nozzle array, and hence this print head is not suitable for a constitution in which a large number of nozzles are disposed at high density.
In Japanese Patent Application Publication No. 2000-289201, drive wiring (an aluminum plug) connecting the piezoelectric element and wiring layer is formed to pass through a laminated plate between the piezoelectric element and wiring layer, which are disposed on either side of the pressure chamber. As a result, it is difficult to secure enough space for the drive wiring and dispose the nozzles at high density.
In the print head disclosed in Japanese Patent Application Publication No. 2003-512211, a common liquid chamber (ink manifold) storing ink to be supplied to the ink supply layer is provided on the opposite side of the wiring layer to a wiring member side, causing a flow passage that connects the common liquid chamber and pressure chamber via the ink supply layer to increase in length. Hence, if the density of the nozzles is increased, there may not be enough time to supply ink from the common liquid chamber to the pressure chamber. In particular, when highly viscous ink is used, the ink supply layer is constituted by a porous member, and therefore ink supply may be delayed even further.
In response to these problems, a patent application which was, at the time the present invention was made, not published, not publicly known, and assigned to the same assignee to which the present invention was subject to an obligation of assignment, proposes a print head in which the common liquid chamber is provided on the opposite side of the diaphragm to the pressure chamber, and a wiring member including wiring for driving the piezoelectric element is provided so as to pass through the common liquid chamber.
It is desirable to improve this print head to prevent deformation of the diaphragm under the load that is applied when the wiring member and piezoelectric element are connected. In other words, if the piezoelectric element or diaphragm deforms under the load that is applied at the time of connection, it may become difficult to obtain the desired ejection performance. Moreover, if an even larger load is applied, the piezoelectric element may break.
In the print head disclosed in Japanese Patent Application Publication No. 2003-512211, the wiring for driving the piezoelectric element, which is provided in a direction substantially perpendicular to the diaphragm, is constituted by an elastic member in order to prevent deformation of the piezoelectric element. However, the disposal of the wiring member is not taken into account, and therefore the piezoelectric element or diaphragm may deform under the load applied during connection.
Furthermore, following completion of the print head, stress generated during incorporation into a housing or the like is applied, via the wiring member, to the piezoelectric element and the diaphragm. Hence, the piezoelectric element and diaphragm may deform as in the case of the time of connection. Even when a manufacturing method which does not require joining based on applying load to the wiring member, such as a manufacturing method using a photo-process, is employed, if the wiring member exists directly above the pressure chamber cavity, the stress on the print head generated during incorporation into a housing is applied to the piezoelectric element and diaphragm via the wiring member, and hence the problems described above may still occur.